1. Technical Field
The present invention relates to a power-on reset (POR) circuit, and more particularly to a zero-power POR circuit.
2. Related Art
A POR circuit is a circuit used to ensure provision of a global reset signal to a system during an initial stage in which powering up of a semiconductor device starts, so as to ensure that the whole device starts in a determined state.
FIG. 1 is a schematic view of a conventional POR circuit. As shown in FIG. 1, at the beginning of application of a power supply to a circuit, a power supply voltage VDD rises from zero along an oblique line, so as to charge a capacitor C through a resistor R. When the capacitor C is being charged, a voltage of a node Va follows the voltage VDD. When Va increases to a certain threshold, a Schmitt trigger 10 reverses, a signal ˜RST is of a high level, and the high level signal ˜RST provides a global reset signal for the system, so as to ensure that the whole system starts in a determined state. Meanwhile, the high level signal ˜RST turns on a PMOS tube 14 through an inverting amplifier 12, so as to short-circuit the capacitor C to VDD. In a case where the power supply voltage is stable, the whole reset circuit consumes no quiescent current, and does not cause power consumption.
However, the power supply voltage may be unstable, and a glitch or a brief decrease may occur, the duration of which may be hundreds of microseconds or may be up to the order of milliseconds. Generally, the RC in the POR circuit is large, and Va decreases very slightly, and cannot decrease to a second threshold that makes the Schmitt trigger 10 to reverse again. Therefore, in the case of a power supply glitch, ˜RST is very likely to be still kept at the high level. For some types of electronic apparatuses, for example portable apparatuses, the apparatuses may be frequently turned on and off, so that VDD decreases briefly. However, when the POR circuit shown in FIG. 1 is adopted, in the case of a power supply glitch or a brief decrease of the power supply voltage, the electronic apparatus cannot be reset normally, so that functions of components may be in chaos.